THE STARS

About 6,000 stars can be seen with the naked eye. The stars indicated in the drawing are those that are less than 160,000 billion km far from the Sun. (JPEG, 100 K)
(Drawing: Michelangelo Miani)

Hipparcos: the star geometer.

The ESA satellite Hipparcos.
Launched on August 8th 1989 by an Arian rocket, Hipparcos measured the position of over a million stars. Its mission terminated in 1993.(JPEG, 292 K)
(ESA)

Where are the stars? To measure the precise position of the stars, their mutual distances and their motion is extremely important for astronomers. Hipparcos was the first satellite in the world to carry out such mission. Developed by the European Space Agency (ESA), and launched by Ariane 4 in 1989, the Hipparcos satellite kept under observation 118,218 stars for four years and realized the first accurate census of the star population of our galaxy. The measurements, published in a catalogue that was recently made available for the international scientific community, are precise to the millionth of a degree, that is a hundred times more than those that can be obtained by observations carried out in the most sophisticated observatories on the Earth. A second catalogue gives the positions and the motions of 1,038,332 stars with a precision of a centimillionth of a degree.

Representation of the Hipparcos satellite with the solar panels spread out, and the apogee engine. Its shape is that of an exagonal prism with sides 1.5x2.2m. (JPEG, 697 K)
(ESA)

The light coming from stars the directions of which are separated by a large angle (green lines), travels to the same focal plane of the telescope. Thus, the apparent movement of a star in the sky when it is observed by different points of the orbit of the Earth around the Sun, can be measured. This is called "parallax effect", and allows to calculate the exact distance of the star. (JPEG, 233 K)
(ESA)

Assembly of the Hipparco satellite and the functioning test carried out at Aeritalia (now Alenia Spazio) in Turin before the launch which took place in 1989. (JPEG, 303 K)
(ESA)

THE BIRTH OF THE STARS

A star originates from the gravitational contraction of a cold cloud of gas and dust. The contraction of the cloud compresses the gas, prevalently constituted by Hydrogen, thus increasing its temperature. When the temperature in the centre reaches 5-6 million degrees, the thermonuclear reactions, which burn the Hydrogen transforming it into Helium, are triggered. The energy that is produced heats up the gas of the cloud that tends to expand, thus blocking the contraction until it stops. A star is born from the cloud of gas.

Columns of Hydrogen and interstellar dust from which new stars are originating, in the Eagle Nebula. This region of stellar formation is 7,000 light years far from the Solar System. (1 light year = 9500.000.000.000 km). (JPEG, 50 K)
(NASA-STScI)

The Orion nebula is the most famous region of stellar formation in our galaxy; it can be seen even with an amateur telescope in the Orion constellation. The top right image shows the nebula photographed by a terrestrial telescope. Bottom left: detail of the nebula studied by the Space Telescope. In the centre you can see very young stars still immersed in the cloud of gas and dust from which they originated. The Orion nebula is 1,500 light years far. (JPEG, 244 K)
(NASA-STScI)

Eta Carinae, a region of intense stellar formation at a distance of 8,000 light years from the Sun. It was observed for the first time in 1677 by the English astronomer E. Halley. (JPEG, 181 K)
(ESO)

Emission of gas by originating stars. The scale in the bottom left angle represents a 1,000 times the distance between the Earth and the Sun. (JPEG, 442 K)
(NASA-STScI)

Stellar formation in other galaxies. New stars are originated also in other galaxies. the NGC 604 stellar formation region lies in a spiral arm of the M33 galaxy, at a distance of 2.7 light years. (JPEG, 478 K)
(NASA-STScI)

THE LIFE OF THE STARS

The triggering of the thermonuclear reactions in the centre of the cloud marks the beginning of the life of a star. Since this moment on, its stability is assured by the balance between two forces: gravitational force, which tends to compress the star under its own weight, and the gas expansion caused by the nuclear reactions that take place in the centre of the star and that transform the Hydrogen into Helium. A star spends about 90% of its life in such state of balance. The Sun, for example, has already lived in this state for 5 billion years and will continue for another 5 billion years before expanding and becoming a red giant.
In the nucleus of the stars, where the temperature reaches very high values (20 million degrees in the Sun), four Hydrogen nuclei can meet and form a Helium atom thus causing the release of a large amount of energy. In a subsequent phase the Helium nuclei will collide forming a Carbon nucleus. In every phase of stellar evolution elements with increased atomic weight are formed. <(De Agostini)>

The colour of the stars is determined by their superficial temperature. A star with a temperature of 40,000 degrees appears blue, a star of 10,000 degrees appears white, a star, like the Sun, of 5,500 degrees yellow, a star of 2,000/3,000 degrees red. The luminosity of a star depends on its temperature and its size. Cold stars like the red supergiants can infact be more luminous that hotter stars like blue supergiants. <(De Agostini) >

The image of Betelgeuse, one of the most brilliant stars of the boreal hemisphere, in the Orion constellation. It is approximately 500 times larger than the Sun. If it were placed in the centre of the Solar System it would spread to reach the orbit of Mars. (JPEG, 260 K)
(NASA-STScI)

THE DYING STARS

Stars do not end their lives in the same way. When the Hydrogen in the centre of the star is finished and the nuclear reactions die out, gravitational force prevails and the nucleus starts to concentrate while the outer layers expand. During this phase the stars with a small mass (up to 8 times the mass of the Sun), begin to lose the coat of Hydrogen that constituted the more external parts of the star, thus forming nebulae with a typical shape and leaving a hot nucleus exposed in the centre, very dense and very small (approximately 10,000 km the diameter and 1 ton per cube cm the density): a white dwarf composed of Helium and Carbon.
The more massive stars (at least 10 times the mass of the Sun) end their life in a catastrophical way, exploding as Supernovae. Among the astrophysical phenomena that can be observed, the explosion of a Supernova is one of the most violent. In a few months a Supernova emits the same amount of energy as that emitted by the Sun in 10 billion years. The explosion usually destroys the star. In some cases a hyperdense "residue" can remain: a neutron star or a black hole. The density of a neutron star is approximately 100 million tons per cube cm.

The life cicle of the stars.
The evolution and the death of a star essentially depends on its mass. A star such as the Sun will end its life expanding and becoming a "red giant" star, the external layers of which will tend to dissolve. Thus, a nebula with a small star in the centre is formed - a white dwarf - that will die out very slowly. If the mass of the star is at least 10 times larger than that of the Sun, it will eventually become a "supergiant" star, which will later explode like a Supernova, sometimes leaving a small and dense neutron star in the centre. If the mass is even larger, the outcome will probably be a black hole. (The mass of the Sun is equal to 2.000.000.000.000.000.000.000.000.000 tons). (JPEG, 107 K)
(Drawing: Michelangelo Miani)

The relative dimensions of the stars. (JPEG, 474 K)

The Helix nebula, 450 light years far, is the scenario of a dying star among the nearest to the Solar System. The central star lost most of the external gaseous matter and became a white dwarf that will slowly die out. (JPEG, 90 K)
(ESO)

The Egg Nebula is composed of the material emitted by a dying star. The central part is now darkened by a "bar" of dust and gas emitted by the star during its evolution. A few hundred years ago the star was completely different: infact, it was a red supergiant. It is situated at a distance of 3,000 light years. rossa. (JPEG, 90 K)
(NASA-STScI)

The sand-glass shape of this nebula is produced by the rapid expansion of the gaseous layers within a cloud of gas that is more dense in the equatorial regions than in the polar regions. Its distance is 8,000 light years. (JPEG, 180 K)
(NASA-STScI)

The shape of this nebula is that of a "cat's eye", and is the result of at least two episodes of matter loss, which happened in different ages in the last 1,000 years. The nebula is situated at a distance of 3,000 light years, and its dimensions are approximately 4,000 billion km. (JPEG, 169 K)
(NASA-STScI)

Detail of the Helix nebula photographed by the Space Telescope. The gaseous nodules are the result of the encounter between the "hot" gas emitted by a dying star and the "colder" gas dispersed in the environment in earlier ages. The size of the smallest nodules that can be seen in the picture is about 12 billion km (2 times the dimensions of the Solar System); they will eventually disperse in the interstellar space. (JPEG, 84 K)
(NASA-STScI)

COSMIC WRECKS

The explosion of a Supernova causes the violent expulsion, with a speed of 10-15,000 km per second, of the most external layers of the star, leaving a hyperdense nucleus: a neutron star or a black hole. The frequency of supernovae explosions for a galaxy such as the Milky Way is about 1 every hundred years. The shock front of the explosion compresses the surrounding interstellar gas favouring the formation of new stars and new planetary systems. The chemical composition of the new stars will be different from that of the preceeding star generation. The new gaseous cloud are infact enriched in "heavy" elements such as Carbon, Oxygen, Nitrogen, Magnesium, Potassium and Iron, formed during the nuclear reactions in the nuclei of massive stars, which later exploded.

The Crab Nebula is the remainder of a supernova explosion. The nebula, 7,000 light years far from the Solar System, is about 10 light years long along the major axis. In the centre of the nebula you can see, indicated by the arrow, the pulsar: the hyperdense nucleus of the massive star exploded about 900 years ago. The pulsar of the nebula of the Crab is a neutron star that rotates 30 times in a second. On the left you can see the image taken from the Earth with the 5 m telescope of Mount Palomar, with the Space Telescope on the right. (JPEG, 283 K)
(NASA-STScI)

The explosion of the 1987 A Supernova. It was observed 10 years ago in the Large Magellanic Cloud, but due to its distance and to the low speed of light (300,000 km/sec in the vacuum), the explosion actually happened 170,000 years ago. (JPEG, 533 K)
(ESO)

The remainder of the explosion of a massive star: the 1987A supernova photographed by the Space Telescope. The rings are the result of the violent expansion of the stellar material. (JPEG, 127 K)
(NASA-STScI)

The remainder of a Supernova explosion, in the Constellation of the Swan, that happened about 50,000 years ago. The expelled gas compresses the surrounding interstellar gas, thus favouring the formation of new stars. (JPEG, 490 K)
(NASA-STScI)

The explosion of a star in the sky and its fast increase in luminosity (up to a million times that of the Sun) have the effect of adding to the sky a "new star" where the naked eye could see nothing before. Since ancient times such stars are called "novae". The nova in the Constellation of the Swan is 7000 light years far and the diameter of the Hydrogen ring around the white dwarf is approximately 150 billion km long. The increase in luminosity is due to the thermonuclear burning of Hydrogen that precipitates on the white dwarf. Such phenomenon happens again on the same star with a frequency of approximately one explosion every 100,000 years, for about 10,000 times. (JPEG, 560 K)
(NASA-STScI)

The brown dwarfs are still mysterious objects, half way between stars and planets. Originated from the contraction of small Hydrogen clouds, they never triggered any thermonuclear reactions due to their very small mass. (JPEG, 112 K)
(NASA-ESA)

The name "black hole" derives from the fact that not even light can escape its gravitational force. Therefore a black hole cannot be observed directly, not even with the most modern telescopes. Its presence can be detected indirectly by studying the effects of its intense gravitational field on the surrounding matter. (JPEG, 544 K)

DO YOU WANT TO KNOW MORE? CLICK HERE !